This invention relates to a method and apparatus for oxidizing and removing remaining sodium sulfide and generated malodorous sulfur compounds by introducing oxygen into the cooking liquor at high temperature and high pressure in the end stage of kraft cooking.
It has already been known in theory that malodorous substances and other potential malodorous substances of kraft cooking can be killed off by introducing oxygen into the cooking liquor. In reality, however, since a decrease of pH of the cooking liquor is effected inherently as a result of the reaction of oxygen with organic substances in the liquor, a measure must be taken to counteract adverse effects resulting from such pH decrease. While the pH decrease in an appreciable degree may help increase pulping yield and is therefore favorable, it promotes undesirable release of methyl mercaptan or the like from liquor phase to gas phase. Consequently, the introduction of oxygen loses much in its merit unless the oxidative removal of free mercaptan or the like in the gas phase is carried out at substantially the same time as oxygen is introduced into the liquor phase.
If the reaction or contact duration of oxygen with the cooking liquor, i.e. the quantity of oxygen introduced, is kept at an optimum level which is close to the minimum, the pH decrease of the cooking liquor is optimumly minimized. While this may deteriorate the favorable yield improvement achieved by the pH decrease, effect of the oxidative removal of malodorous substances can be enhanced to a maximum degree and the cost for oxygen can be also reduced.